Laser produced plasma (LPP) has been extensively investigated as a potential high power light source for the Extreme Ultraviolet (EUV) lithography industry. Generally in LLP, a high power infrared laser is used to irradiate a metal or gas target to generate plasma emitting EUV radiation, and the radiation is then collected by an EUV collector mirror. Similar concepts have been used to develop broadband UV-VIS light sources, specifically laser-driven light sources (such as LDLS™, commercially available from Energetiq Technology Inc., Woburn, Mass.).
The LDLS concept is very similar to LPP as a source for EUV radiation. A focused high power infrared (IR) laser passes through a hole at the center of a collector mirror. This configuration is due to lack of an IR transparent and DUV-VIS high reflective mirror, or a UV-DUV expanded cold mirror. As an alternative to a conventional cold mirror, aluminum-based mirror have been used. However, the aluminum-based mirror is opaque in the IR range, and thus does not transmit light from the IR laser. As a result, a center hole in the mirror is commonly necessary in order to allow passage of light from the IR laser. In addition, the aluminum-based mirror has a low damage threshold which makes the aluminum-based mirror less compatible with high power light sources.
Cold mirrors are frequently applied to reflectors that are located close to a source. FIG. 1A illustrates a cold mirror having an angle of incidence of 0 degrees. FIG. 1B, is a graph of reflectance versus wavelength for the cold mirror of FIG. 1A, shows that the cold mirror has high reflectance at wavelengths of about 400 nm to about 650 nm when operating in the VIS-IR range. Similarly, FIG. 1C illustrates a cold mirror having an angle of incidence of 45 degrees. FIG. 1D, which is a graph of reflectance versus wavelength for the cold mirror of FIG. 1C, shows that the cold mirror has high reflectance at wavelengths of about 400 nm to about 650 nm when operating in the VIS-IR range. Such cold mirrors are designed to reflect about 90% of visible light and to transmit about 80% of infrared radiation/energy (heat) and may be used to provide visible light for lighting applications, such as lighting for stadiums, projection lighting, studio settings, medical applications and others, while also reducing the amount of reflected heat.
Although a conventional cold mirror can be modified to provide high transmittance in the NIR range, the high reflective band in the DUV range of the mirror is limited. For example, as shown in FIG. 2, the high reflective band in the DUV range for a conventional cold mirror operating at an angle of incidence of 0 degrees is about 15 nm. Similarly, as shown in FIG. 3, the high reflective band in the DUV range for a conventional cold mirror operating at an angle of incidence of 45 degrees is about 10 nm.